Decoking of onstream thermal cracking tubes

ABSTRACT

THERMAL CRACKING OF HYDROCARBONS IN ADMIXTURE WITH STEAM IN TUBES ARRANGED IN A CRACKING FURNACE LEADS TO THE DEPOSITION OF COKE ON THE INTERIOR WALLS OF THE TUBES, WHICH COKE MUST BE PERIODICALLY REMOVED IN ORDER TO MAINTAIN CRACKING EFFICIENCY; THE COKE CAN BE REMOVED, WITHOUT THE NECESSITY OF SHUTTING DOWN THE FURNACE, BY CUTTING OUT THE COKING HYDROCARBON FEED TO AT LEAST ONE TUBE AND PASSING THROUGH SUCH TUBE OR TUBES A DECOKING FEED OF STEAM AND A HYDROCARBON WHICH WHEN CRACKED WILL PRODUCE HYDROGEN AS A BY-PRODUCT, THE DECOKING FEED BEING SUBSTANTIALLY SULFUR FREE, WHILE MAINTAINING THE FURNACE ONSTREAM AND CONTINUING THE THERMAL CRACKING PROCESS IN THE TUBES THAT ARE NOT BEING DECOKED.

Feb. 8, 1972 J. A. KIVLEN ETAL 3,641,190

DECOKING 0F ONSTREAM THERMAL CRACKING TUBES Filed Jan. 22, 1969 2 Sheets-Sheet 1 3y JW Attorney Feb. 8, 1972 J. A. KIVLEN ETAL 3,641,190

I DECOKING OF ONSTREAM THERMAL CRACKING TUBES Filed Jan. 22, 1969 2 Sheets-Sheet 2 FIGURE 2 COKING RATE IN ETHANE CRACKING AS A FUNCTION OF SULFUR CONTENT 2 2 I I I I I GRAMSOF COKE/2 I I I I I I o 20o 400 600 800 |000 |200 |400 wT. PPM Has FIGURE 3EFFECT OF TRACE SULFUR ON THE DECOKING RATE |.0 I I I I I [9 VPPM H25 ADDED CC CO/ SEC O 40 8O l |20 |60 200 240 TIME, MINUTES e Wm Inventors United States Patent O U.S. Cl. 260.-683 27 Claims ABSTRACT OF THE DISCLOSURE Thermal cracking of hydrocarbons. in admixture fwith steam in tubes arranged in a cracking furnace leads to the deposition of coke on the interior wallsof the tubes, which coke must be periodically. removed in order to maintaincracking efiiciency; the coke can be removed, without the necessity of shutting down the furnace, by cutcrease coking rates thereby adding to the over-all prob.

ting .outlthe coking hydrocarbon feed to atleast one tube and passing'th'rough such tubeor tubes a decoking feed ofrste'am and a hydrocarbon which when cracked ,will

produce hydrogen vas ya by-product, vthe decoking feed being substantially sulfur free, while maintaining the furnace onstream and continuing the thermal cracking process in the tubes that are not being decoked.

CROSS REFERENCE TO RELATED APPLICATIONS' This application is a continuation-in-part of Ser. No. 554,239 filed Apr. 29, 1966, now abandoned, which in turn'i's a continuation-impart of Ser. No. 407,569 led Oct. 29, 1964, now abandoned.

FIELD OF THE INVENTION lThis invention relates to a process for decoking the tubes of a cracking furnace. More particularly, this invention relates'to an improved process for decoking` steam cracking tubes while maintaining the furnace onstream and continuing thecracking' process. Still more particularly, this invention relates to an onsltream decoking processl whereby the initial hydrocarbon feed to at least one tube inthe cracking -furnace is eliminated and a decokin'gfeed ofdsteam and a substantially sulfur free hydrocarbon, which when" cracked will produce hydrogen, is passed through the' tube or tubes to be decoked, thereby removing the coke from those tubes, and thereafter returning those tubes to normal operation.v

PRIOR ART The thermal cracking of petroleum feed streams with or without the presence of steam is well known to the art and is Widely used as a source of valuable unsaturated compounds, c g., ethylene, propylene, butadiene, as well asfhydrogen.l Generally, when noncatalytic processes are conducted, it'iis desirable to employ steam `as the principal diluent'z'in order to control the reaction and reduce erosion and corrosion effects. While steam cracking has been technicallyand economically successful, several major drawbacks exist which militate againstv the development of thefull potential of the steam cracking process. Essentially, these drawbacks center around a secondary reaction, i.e., Vthe carbon forming tendency of the Vaporized petroleum'feed atr reaction (cracking) temperatures.

Perhaps the most objectionable drawback relating to carbon formation is the deposition of coke on the interior of the tube`walls through which the cracking mixture flows.` Coke deposition is believed to be `due to the formation of free radicals, e.g.when ethane isV` cracked meth- 3,641,190 Patented Feb. 8, 1972 ICC ylene radicals can be formed, which may then polymerze with other unsaturated components into long chain compounds and dehydrogenated to form coke on the tube walls. (Additionally, sul-fur, in the form of elemental sulfur, hydrogen sulde, organic suliides, etc., which is present in virtually all steam cracking feeds tends tol inlems.) The rate of coking, which varies with the typeof feed employed is nevertheless continuous and, therefore, l the coke builds up and reduces the eiective cross-sectional area of the tube thereby necessitating higher pressures to maintain a constant throughput. More importantly,.how ever, because coke is an excellent thermal insulator, its

formation on tube walls must be accompanied by a sharp increase in furnace tube temperature in order to maintain cracking eiciency, i.e., to keep the reactants at the desired cracking temperature. Operating with high tube metal temperatures, however, results in a decrease in tube life which in turn limits the practical cracking temperature that can be employed (also limiting ultimate conversion and yield). Eventually, the coke. buildup is such that the furnace must be shut down for decoking with a consequent loss in production capacity.

The coking problem has now been attacked by a processwhich basically involves the elimination of the hydro- SUMMARY OF THE INVENTION In accordance with this invention, therefore, an improved process for the decoking of tubes arranged in a cracking furnace is provided whereby the initial or original hydrocarbon feed (having a coking tendency) is eliminated from one or more tubes in the furnace and a decoking feed comprising steam and a substantially sulfur free hydrocarbon which produces hydrogen upon cracking is passed through the tube or tubes from which the initial feed has been eliminated while maintaining the remaining tubes in normal service, effecting the removal of coke from the tube walls in tubes where the decoking feed is passed, and thereafter returning the decoked tubes to normal service. This invention has the advantage, among others, of lending a decoking capability to the steam cracking process which allows decoking of one or more tubes while maintaining furnace temperatures and continuing to make product. Additionally, this invention contemplates the decoking of only a single tube at a time in the furnace, or the decoking of any number of tubes either simultaneously, or successively, the decoking of a minor portion, major portion, or all of the tubes of the furnace simultaneously. After the decoking operation has been completed, the clean tubes are returned to normal service by reintroducing the initial hydrocarbon feed.

While not wishing to lbe bound by any particular theory, it is believed that the decoking reaction results from an interaction of steam and coke, according to Equation l:

i.e., the water gas reaction. However, at steam cracking temperatures this reaction is quite slow and would not be expected to be economically feasible as decoking method. Nevertheless, it is believed that as the coke forms on the tube surfaces a diffusion process takes place between the coke and the metal (generally ferriginous) tube.

Thus, some coke goes into solid solution in the metal tube while trace amounts of metals, e.g., chromium, nickel, iron, etc. (nickel and chromium being present in substantial amounts in the widely used stainless steel tubes), diffuse into the coke layer. These trace amounts of metals are then believed to catalyze the water gas reaction allowing it to proceed at favorable rates at steam cracking temperatures. Additionally, it is believed that there is an induction period during which the water gas reaction does not proceed rapidly. This induction period is believed to be caused by the presence of sulfur (which is present in some amount in just about every steam cracking feed) in the coke which effects the trace elements thereby masking their catalytic effect. Only when the sulfur is removed do the trace elements become sufficiently active to catalyze the water gas reaction. Consequently, in order to obtain practical decoking rates, desulfurization of the coke becomes necessary. Of course, desulfurization can only be effected when the sulfur content of the feed is sufficiently low so as to prevent sulfur from becoming absorbed (or diffused) into the coke layer on the tubes. Thus, in the process of this invention a decoking feed which is substantially sulfur free is employed. The decoking feed, as mentioned, contains steam (for the water gas reaction) and a hydrocarbon which when cracked will produce hydrogen. When the saturated hydrocarbon is cracked hydrogen is produced as a by-product:

CzHe A+ 02H4 -l- H2 -lother ley-products (2) The hydrogen by-product then acts as a desulfurization agent:

HZ-l-S (in coke or adsorbed on Ni or Fe)- H2S-l-desulfurized coke-l-desulfurized Ni or Fe (3) Since it is believed that the thermal crackin of any hydrocarbon feed produces some coke, the by-products produced in Equation 2, above, will contain coke. Now, the decoking cycle can be looked at as two separate, overlapping reactions: (i) the desulfurization of the coke to expose the active sites for catalyzing the water gas reaction, and (ii) the water gas reaction to remove the Coke. However, throughout both reactions, coke is continuously being produced due to the cracking of the hydrocarbon in the decoking feed. Nevertheless, by utilizing as the decoking feed the hydrocarbons mentioned herein, the gasification reaction occurs at a faster rate than does the coke forming reaction (of the decoking feed) and the coke can be removed.

While it is diflicult to determine with exactness the characteristics of the feed which will lead to decoking, the fact remains that the decoking feed must be one which is capable of removing coke from tube walls. Thus, one skilled in the art, from the directions given herein, will be readily able to determine suitable decoking feeds and, moreover, to determine which feeds are decoking and which feeds are coking.

Now, the general characteristics of a decoking feed are that it contain (i) steam/water in amounts ranging about -95 mole percent, preferably about 50 to 95 mole percent and more preferably about 70 to 95 percent; (ii) a hydrocarbon which when cracked at furnace temperatures will produce hydrogen (cf. Equation 2 above); and (iii) be substantially sulfur free.

With regard to characteristic (ii), as previously mentioned, all hydrocarbons produce some coke upon cracking. The hydrocarbon must be one which will permit the water gas reaction to be dominant and, therefore, effect coke removal. However, the hydrocarbons mentioned herein as being useful in decoking can be used satisfactorily in this invention.

With regard to characteristic (iii), it is known that the presence of sulfur in any feed stock will lead to the formation of coke. Nevertheless, it has now been discovered that a sulfur free or substantially sulfur free feed can be used to decoke steam cracking tubes. Preferably, the sulfur level should be no more than about 3 p.p.m. in the feed, preferably no more than about 2 p.p.m., and still more preferably no more than about l p.p.m., based on hydrocarbon. Sulfur levels in a feed stock in excess of these levels will generally tend to promote coking rather than decoking.

Generally, the hydrocarbon portion of the decoking feed can be characterized as any compound that will produce hydrogen when cracked by the application of heat. This invention contemplates using a hydrocarbon for decoking that is different from the initial feed or the same as the initial feed so long as it is sulfur free. Thus, a coking hydrocarbon feed could be passed through a desulfurization unit and then used as the decoking hydrocarbon feed. However, saturated hydrocarbons, particularly the n-paraflins, are preferred since these compounds tend to maximize the hydrogen produced by cracking, relative to the other cracking products. Still more preferably, saturated hydrocarbons having an increasing hydrogen to carbon atomic ratio, i.e., compounds of decreasing carbon number (methane excepted) are employed for maximum hydrogen production. Additionally, the hydrocarbon portion of the decoking feed should coke at a rate consistent with the gasification reaction so as to allow coke removal. Examples of compounds that can be used herein are saturated hydrocarbons, cyclic and acyclic, as well as unsaturated hydrocarbons both cyclic and acyclic, and substituted or unsubstituted aromatics. Specifically, and preferably, saturated cyclic or straight chain hydrocarbons are employed. (Branched paraiiins can also be employed but compounds that have structures which would readily -be cracked with the formation of free methylene radicals, e. g., isobutane, are not desirable. These latter type compounds can have a coking rate which is inconsistent With coke removal or gasification.) Some compounds that can be employed are cyclopropane, cyclopentane, cyclooctane, etc.and mixtures thereof; acyclics such as any alkane, e.g., aliphatic hydrocarbons of the methane series, or mixtures of alkanes with cycloalkanes. Preferred saturated hydrocarbons for use in decoking feeds are straight chain compounds having from 2 to about 24 carbon atoms, preferably 2 to about l2 carbon atoms and more preferably about 2 to 6 carbon atoms. Examples of such compounds are: ethane, propane, butane, n-hexane, n-decane, n-dodecane, n-hexadecane, eicosane, and tricosane. Additionally, naphthas, gas oils boiling in the range of from about 450 to about 800 F. and higher, and kerosenes boiling in the range of about 430 to about 550i F. can also be used herein but are less preferred. Ethane, however, because of its ready availability and high hydrogen to carbon ratio is most preferred.

The advantages of the decoking method described herein are now abundantly clear. Firstly, the furnace need not be shut down with its attendant loss in productivity. Secondly, decoking can be carried out in one or more tubes while the remaining tubes continue to crack the original feed. Thirdly, and perhaps most importantly, product is made even in those tubes that are being decoked. Thus, steam cracking is a primary process for the production of oleins, such as ethylene. As can be seen from Equation 2 above, ethylene is produced from the substantially sulfur free hydrocarbon employed as a deco-king feed. Consequently, the production capacity of the furnace stays undiminished even during the decoking period.

DRAWING DESCRIPTION FIG. 1 shows a typical steam cracking process flow path.

FIG. 2 shows the increase in coking rate as function of increasing sulfur content in the feed.

FIG. 3 shows the decoking rate reported as carbon monoxide in the coil outlet as a function of time.

Referring to the drawing, the cracking furnace 10 comprises an upper convection or preheat section 11 and a lower` cracking zone 12. Burners 13 are provided on the sidewalls and/or on the bottom of the furnace to supply heat. The number of Iburners provided is dependent upon theheat required and may vary considerably.

Although not shown in detail in the drawing, thefurnace contains several conduits or passes in parallel. Each pass may contain a number of connected tubular members or tubes that provide a flow path through the convectionsection and into the cracking section. In the drawing,v one pass is shown, with the tubes in the convection `section 11 designated by the numeral 15 and the cracking coils or tubes in the cracking zone 1 2 designated by the numeral 16. It is to be understood that the number of conduits or tubes inthe furnace is a function of the size` of the furnace, and is dictated solely by design considrations. l

Petroleum feed stock is supplied to the steam cracker via supply conduit and manifold or distributor conduit 21 to the several parallel cracking conduits or passes. A control valve 22 is provided on each conduit 23 connecting the feed distributor 21 to each of the cracking conduits or tubes. Steam is supplied through inlet line 24 and valve 25 to the conduit 23. (In some cases, steam and water are supplied through separate lines and not necessarily at the identical point in the convection section.)

The reaction products are discharged from the coils or tubes 16 of the cracking furnace via `conduits 26 into conduit orE header 27 from whichl they are discharged into conduit 28. In order to stop the cracking reaction promptly and thereby prevent or minimize side reactions, quenching vagents such as higher boiling hydrocarbons and/ or vwater are supplied through conduit 29 and control valve 30. Alternately, the reaction may be stopped by cooling the furnace efiiuent in an exchanger while generati-ng steam directly or indirectly. The mixture of quenched reaction products and quenching agent are discharged via conduit 28 into fractionating tower 31. Amatic tar product is withdrawn from the bottom of fractionating tower 31 through line 32 and product .is taken overhead via line 33. Other intermediate boiling range fractions may be withdrawn as product or recycled to a higher plate in the' fractionating tower as one or more reflux streams. The quench oil may be withdrawn from the fractionating tower 31 through line 34 and passed through heat exchanger 35 where it is passed in indrect'heat exchange relation to the hydrocarbon feed.

stockforpreheat thereofor to water for steam formation while cooling the quench oil to a suitable'temperature for discharge through line 29 and valve 30 into the reaction product stream in line 28 as described above.

'The onstream decoking procedure requires the closing of one or more of the hydrocarbon feed valves 2 2 and the opening of :the `corresponding number of alternate hydrocarbonl feed Valves 37. The essentially sulfur free hydro-V carbon, i.e., a light hydrocarbon feed containing less than 3 parts of sulfur per million parts of hydrocarbon, passed through the alternatefeed conduit 36 is adjusted so that the alternate hydrocarbon feed is cracked to a reasonable conversion level normally producing between 15 to 60% ethylene.v The outlettemperatures are usually between 1400 andfl9800 F. It is also within the scope of this invention to decoke two or more coils simultaneously by admitting substantially sulfur free hydrocarbon decoking feed through a plurality of Vcracking coils such as -that shown in the drawing by element 15. When sufficient time has-elapsed to allow the coke to be removed from the inside of the tubes, valve 36 is closed and valve 22 is opened.

-When decoking is completed, e.g., about 2 to 8 hours,

sometimes up 'to l0 hours or more, for a typical tubel under optimum conditions, the decoking feed is cut out and the original feed is reintroduced. The completion of the decoking operation can be monitored by anyone of several methods, such as (i) decrease in pressure drop acrossthe section of the furnace to be decoked, (ii). de-

crease in tube metal temperature, or (iii) rate of carbon monoxide formation (cf. IEquation 1 above, CO will increase during decoking but fall off sharply when little or no coke is left in the tube).

FIG. 2 depicts a graphical portrayal of increase in coking rate due to the presence of sulfur (as hydrogen sulfide) in an ethane feed. The data for this figure were- FIG. 3 is a graphical portrayal of the rate lof decokingj as a function of time and typical of a sulfur free ethane decoking feed. This graph shows that the decoking rate (as measured by carbon monoxide make) startsoli` at a low level (induction period) and gradually increases until about 160 minutes of operation when the decoking rate started to level off. At that point, 1.9 volume ppm'. of hydrogen sulfide was added to the feed and the decoking rate dropped off sharply before leveling out at a significantly lower rate. Thus, the effect of sulfur on the decoking rate yis clearly shown by this figure. Furthermore,

it also shows that increasing sulfur content will reduce' the rate of decoking.

The steam cracking operation is old and well known (see, for example, Chemical Week, Nov. 13, 1965, page 72 et seq.), and will only be briefly described hereinbelow. Generally, the hydrocarbon feed fraction is admixed with steam, i.e., in amounts ranging from about 20-95 mole percent steam, prior to entry into the steam cracking furnace which may be heated by any suitable means, eg.,r gas firing, etc. The furnace itself normally contains two sections, a convection section wherein the feed is vaporized, if not already in that form and preheated, and a radiant or cracking section, the feed being passed in admixture with steam through one or more furnace tubes located within the furnace. The convec-v tion section is normally employed to increase heating eiciency and the petroleum-steam mixture is heated therein to intermediate temperatures, i.e., about 1000" to 1l00 F. However, these temperatures are below that at which the feed cracks since cracking is undesirable in the convection section. The heated feed then passes into the radiant section, i.e., the cracking zone, where the temperature of the reactants is quickly raised to about 12007 to 1700* F., preferably 1500 to l700 F., or higher, as tube metal materials permit, and the feed is cracked. (Generally, raising the temperature of the reactants to the mentioned ranges requires heating the tubes to about 1400 to 2000 F., preferably 1600 to 2000 F. and higher as tube materials permit.) Residence times in the radiant section are carefully controlled to minimize polymerization and other undesirable reactions. Thus, residence times in the cracking zone will rangev from about 0.1 to 10 seconds, preferably 0.1-2 seconds, more preferably 0.2 to 0.6 second. Pressures within the tubes may range from about 0 to 50 p.s.i.g. but are not critical, and higher pressures, e.g., up to about p.s.i.g. can be tolerated. Upon exiting the cracking zone, the reaction products are immediately quenched to stop further reaction and/or minimize loss of primary conversion products. (This invention is also applicable to single zone furnaces where the temperature is steadily raised to the cracking point.)

Now, since cracking occurs only in the radiant zone of a two zone furnace, it is only this zone that requires decoking. And, since the flow of decoking feed through this zone will be such as to maintain normal onstream temperatures, the tubes not being decoked can continue to crack feed with little or no disruption to the entire unit.

The hydrocarbons which can be cracked by this process can vary rather Widely and are generally indicated by those hydrocarbons mentioned above which can be used as decoking feed components. However, the nature of the original feed stock can be much broader since it may also contain a wide variety of branched hydrocarbons which are not generally preferred as decoking components.

Example 1 To a furnace containing several banks of continuous nickel-chromium alloy tubes, said tubes being thoroughly coked, was continuously passed through a convection section of the furnace and then to a cracking section of the furnace where the tube is in direct contact with a plurality of direct tired burners, 160 parts per hour of ethane and 50 parts per hour of water. In the radiant section of the furnace the coil inlet temperature of the feed was maintained at l330 F. and the coil outlet temperature was maintained at l 80 F. The outlet pressure was 10 pounds per square inch and there was maintained a velocity at the outlet coil of about 1000 feet per second. The ethane feed contained less than 2 parts of sulfur per million parts of ethane. Carbon in the cracking section of the furnace was reduced or completely eliminated.

Example 2 Example l was repeated except that the ethane contained above 3 parts of sulfur per million parts of ethane. Little or no coke was removed during this operation.

Example 3 Example was repeated except that sulfur free ethane was utilized as the feed. All components of the furnace were completely free of coke.

Example 4 Example 1 was repeated except that an 8 pass furnace normally utilizing cracking gas oil as the feed was completely decoked, one pass at a time, while maintaining normal feed throughputs and conversions in the remaining 7 passes.

What is claimed is:

1. In a process for thermally cracking hydrocarbon materials having a coking tendency by passing the same in a mixture with steam through one or more tubes arranged in a cracking furnace, which process results in the formation of coke deposits on the interior surfaces of the tubes, the improvement which comprises taking at least one tube oifstream by cutting out the flow of the hydrocarbon having a coking tendency through the tube, passing a decoking feed containing steam and a substantially sulfur free saturated hydrocarbon through the offstream tube in an amount suicient to effect the removal of the coke deposits on the interior surfaces of the tubes and thereafter cutting out the flow of the said decoking feed through the tube and returning the said mixture of steam and hydrocarbon material having a coking tendency to the said tube.

2. The process of claim 1 wherein the hydrocarbon contained in the decoking feed contains no more than about 3 p.p.m. sulfur.

3. The process of claim 1 wherein the tubes arranged in the cracking furnace are heated to temperatures ranging from about 1400 F. to about 2000 F.

4. The process of claim 1 wherein a minor portion of coked tubes are taken oifstream and decoked.

5. The process of claim 1 wherein all of the coked tubes are taken oifstream and decoked.

6. In a process for thermally cracking hydrocarbon materials having a coking tendency by passing the same in a mixture with steam through one or more tubes arranged in a cracking furnace, which process results in the formation of coke deposits on the interior surfaces of the tubes, the improvement which comprises taking at least one tube ofstream by cutting out the ilow of hydrocarbon materials having a coking tendency and passing a decoking feed containing steam and a saturated hydrocarbon having no more than about 3 p.p.m. sulfur through the otfstream tubes in an amount sufcient to effect the removal of the coke deposits on the interior surfaces of the offstream tube without deleteriously affecting the amount of desirable product formed by the thermal cracking process.

7. The process of claim 6 wherein, after decoking, the passage of decoking feed to said oifstream tubes is cut out and the said mixture of steam and hydrocarbon material having a coking tendency is returnedto said oifstream tubes.

8. The process of claim 6 wherein the feed hydrocarbon contains more than about 3 p.p.m. sulfur.

9. The process of claim 6 wherein the decoking hydrocarbon contains no more than 2 p.p.m. sulfur.

10. The process of claim 6 wherein the decoking hydro.- carbon contains no more than about one p.p.m. sulfur. f

11. In a steam cracking process which comprises passing a mixture of hydrocarbon materials having a' coking tendency and steam through a tube or tubes arranged in a steam cracking furnace, the tubes being heated to a temperature in the range of about l600 F. to about 2000 F. and the hydrocarbon materials contain more than about 3 p.p.m. sulfur, the improvement which com. prises taking at least one tube olfstream by cutting out the flow of hydrocarbon materials having a coking tendency and passing a decoking feed containing steam and a CZ-C saturated, straight chain hydrocarbon having no more than about 3 p.p.m. sulfur through the oifstream tubes in an amount sufiicient to effect the removal of the coke deposits on the interior surfaces of the oifstream tubes without deleteriously affecting the amount of desirable product formed by the steam cracking process, and thereafter returning the said mixture of steam and hydrocarbon materials having a tendency to the otfstream tubes.

12. The process of claim 11 wherein the decoking hydrocarbon is ethane.

13. The process of claim 11 wherein the decoking hydrocarbon contains no more than about 2 p.p.m. sulfur.

14. The process of claim 11 wherein the decoking hydrocarbon contains no more than about one p.p.m. sulfur.

1S. The process of claim 11 wherein the decoking hydrocarbon is sulfur free.

16. The process of claim 11 wherein one is taken offstream for coke removal.

17. The process of claim 11 wherein several tubes are taken otfstream and the decoking feed is passed through the offstream tubes in succession.

18. T he process of claim 11 wherein all of the tubes are taken olfstream and the decoking feed is passed through all the tubes simultaneously. v

19. The process of claim 11 wherein the decoking period ranges from about 2 to about 10 hours.

20. In a process for thermal cracking of a feed cornprising normally liquid hydrocarbons in a tubular furnace wherein substantial coking ofthe furnace occurs, the improvement which comprises cyclically cracking said feed comprising normally liquid hydrocarbons' Iand ethane, each cycle comprising a lirst time period during which said feed comprising normally liquid hydrocarbons is cracked and a second time period during'which ethane is cracked, the cracking of said normally liquid hydrocarbons and ethane being conducted in the presence of 20 to 95 mole percent steam and at a temperature of from about 1200 F. to about 1700 F. and at a pressure of from about 0 to about 100 p.s.i.g.

21. In a process for thermally cracking hydrocarbon materials having a coking tendency by passing the same in admixture with steam through one or more tubes arranged in a cracking furnace, which process results in the formation of coke deposits on the interior surfaces tube at a time of the tubes, the improvement which comprises taking at least one tube ofstream by cutting out the flow of the hydrocarbon having the coking tendency through the tube, passing a decoking feed containing steam at a substantially sulfur free saturated hydrocarbon different from said hydrocarbon material having a coking tendency through the olfstream tubes in amounts suicient to elect the removal of coke deposits on the interior surfaces of the tubes and thereafter cutting out the flow of decoking feed and replacing the same with the said mixture of steam and hydrocarbon material having the coking tendency.

22. The process of claim 21 wherein the hydrocarbon component of said decoking feed is a saturated straight chain hydrocarbon.

23. The process of claim 22 wherein the hydrocarbon component of said decoking feed contains no more than about 3 p.p.m. sulfur.

24. The process of claim 21 wherein the hydrocarbon component of the decoking feed contains no more than about 2 p.p.m. sulfur.

25. The process of claim 24 wherein the hydrocarbon References Cited UNITED STATES PATENTS 2,168,840 8/1939 Groll 260-683.3 2,215,950 9/1940 Young 260-683 2,218,495 10/1940 Balcar 260-683 2,621,216 12/1952 White 260-683 3,433,731 3/ 1969' Oliver 208-48 DELBERT E. GANTZ, Primary Examiner C. E. SPRESS'ER, JR., Assistant Examiner U.S. Cl. X.R. 

